10:00 AM - 10:15 AM
[SCG46-14] Fluid migration processes in oceanic plates based on vein mapping in the Oman Ophiolite
Keywords:Oceanic plate, Permeability, Oman Ophiolite, Vein property, Mineral vein
Hydrothermal circulation beneath the seafloor occurs widely around ocean ridges, and such fluid movement is dominated by fractures within oceanic plates. Although laboratory experiments have been conducted to measure permeability and other parameters to observe fluid movement within oceanic plates, differences in measurement scales have not allowed us to observe the effects of the relatively large fractures that dominate fluid movement inside the actual oceanic plates. Therefore, not only laboratory-scale results, but also field-scale observations of macroscopic fractures are necessary to estimate fluid movement processes in the actual oceanic plate interior. Therefore, in this study, we conducted field-scale vein mapping in the Oman Ophiolite. The goal is to characterize fractures preserved as mineral veins that used to supply water.
We conducted the study in the Wadi Haymiliyah section of the Oman Ophiolite, a geologic structure where an oceanic plate has been uplifted onto land. In this section, basalt to harzburgite is distributed as outcrops along a continuous dry river. The vein mapping, which is a fluid pathway, measured mineral species, density distribution, vein aperture, and orientation using the scanline method. We counted a total of 510 veins from the top of the oceanic plate to the mantle in this survey. The veins in the oceanic crust are classified into those composed of epidote and amphibole, which formed near the oceanic ridges, and those composed of prehnite, which formed farther from the ridges than these. The mantle section veins were composed of low temperature serpentine. High-temperature veins of near-oceanic ridge origin in the crust showed vein density of 2.5 m-1, aspect ratio of 2×10-3, and the orientation of the veins was parallel to the ridge axis in most cases. Low-temperature veins composed at a distance from the ridge showed vein density of 3.6 m-1 and an aspect ratio of 1×10-3, and the direction of the veins was almost random. Mantle-section serpentinite veins showed vein density of 4.6 m-1 and an aspect ratio of 3×10-3, and many of them were parallel to the Moho.
We determined the permeabilities from the average vein density and aperture. The maximum permeability due to High-temperature veins was ~10-9 m2 and tended to decrease with depth. The maximum permeability due to Low-temperature veins was ~10-10 m2, with random values in the crust. The maximum permeability due to the serpentine vein in the mantle section was ~10-8 m2, higher than that due to veins in the crust, suggesting that the fluid migrates well through the crust into the mantle. These permeabilities are several orders of magnitude higher than those measured at laboratory scales, which may depend on the difference in fracture aperture at different measurement scales, indicating that large-scale fractures dominate fluid migration in the oceanic plate.
We conducted the study in the Wadi Haymiliyah section of the Oman Ophiolite, a geologic structure where an oceanic plate has been uplifted onto land. In this section, basalt to harzburgite is distributed as outcrops along a continuous dry river. The vein mapping, which is a fluid pathway, measured mineral species, density distribution, vein aperture, and orientation using the scanline method. We counted a total of 510 veins from the top of the oceanic plate to the mantle in this survey. The veins in the oceanic crust are classified into those composed of epidote and amphibole, which formed near the oceanic ridges, and those composed of prehnite, which formed farther from the ridges than these. The mantle section veins were composed of low temperature serpentine. High-temperature veins of near-oceanic ridge origin in the crust showed vein density of 2.5 m-1, aspect ratio of 2×10-3, and the orientation of the veins was parallel to the ridge axis in most cases. Low-temperature veins composed at a distance from the ridge showed vein density of 3.6 m-1 and an aspect ratio of 1×10-3, and the direction of the veins was almost random. Mantle-section serpentinite veins showed vein density of 4.6 m-1 and an aspect ratio of 3×10-3, and many of them were parallel to the Moho.
We determined the permeabilities from the average vein density and aperture. The maximum permeability due to High-temperature veins was ~10-9 m2 and tended to decrease with depth. The maximum permeability due to Low-temperature veins was ~10-10 m2, with random values in the crust. The maximum permeability due to the serpentine vein in the mantle section was ~10-8 m2, higher than that due to veins in the crust, suggesting that the fluid migrates well through the crust into the mantle. These permeabilities are several orders of magnitude higher than those measured at laboratory scales, which may depend on the difference in fracture aperture at different measurement scales, indicating that large-scale fractures dominate fluid migration in the oceanic plate.